(1) Field of the Invention
The present invention relates to an image forming apparatus and an image forming method, in particular to a technique for reducing power consumption and first print time.
(2) Related Art
For image forming apparatuses such as printers, conventionally, a reduction in the so-called first print time, which is a period of time from the input of a print instruction by a user to the discharge of the printed recording sheet, and a reduction in power consumption have been demanded in viewpoint of improved user convenience and energy saving. In response, a considerable number of techniques have been proposed to meet these needs.
For example, one structure of a fixing part which fixes an unfixed image formed on a recording sheet to the recording sheet by heat and pressure, secures a fixing nip by pressing a heater against a pressure roller across the fixing belt, maintains the heater OFF until receiving a print instruction, and upon receiving the instruction, starts raising the temperature of the heater and starts performing print operations at the same time. With this structure, the temperature of the fixing nip can be instantly raised to the fixing temperature required to fix unfixed images, and consequently, the first print time can be shortened compared to the structure which takes time to raise the temperature of the fixing nip, and the power consumption can also be reduced compared to the structure which applies power to the heater to maintain a predetermined standby temperature until receiving a print instruction.
Another known structure, although maintaining the fixing nip at the standby temperature until receiving a print instruction, calculates, upon receiving the instruction, an estimated temperature rise time required for the fixing nip to reach the fixing temperature from the standby temperature, and an estimated fixing nip arrival time required, from the start of the sheet feed from a sheet feed tray, for the leading end of the sheet to reach the entrance of the fixing nip, and controls operations of each part such that the timing at which the leading end of the recording sheet reaches the entrance of the fixing nip and the timing at which the temperature of the fixing nip reaches the fixing temperature coincide with each other.
Without such a control, for example, in a case where the estimated temperature rise time is very short and thus the fixing nip immediately reaches the fixing temperature, if the heater is turned ON simultaneously with the start of the feeding of the recording sheet, the temperature of the fixing nip reaches the fixing temperature before the leading end of the recording sheet reaches the fixing nip, causing the heater to be kept ON to wastefully maintain the temperature of the fixing nip at the fixing temperature during the time which is required for the leading end of the recording sheet to reach the fixing nip and is irrelevant to the fixing. On the other hand, as described above, by performing the control which allows the timing at which the leading end of the recording sheet reaches the entrance of the fixing nip and the timing at which the temperature of the fixing nip reaches the fixing temperature to match each other, application of power to the heater becomes unnecessary during the time irrelevant to the fixing, and the power consumption can be suppressed accordingly.
However, even if the above-described control for matching the timings is performed, when the recording sheet includes a portion without a formed image, that is to say, a non-image area from the leading end, in the recording sheet convey direction, of the recording sheet to the leading end of the image-formed area, the non-image area is also heated by the fixing temperature. This causes the wasteful consumption of power due to heating the non-image area, which is not required to be heated in the first place, at the fixing temperature.